1. Field of the Invention
The present invention relates to a movable head position controlling device which is used for magnetic recording and reproducing apparatuses such as a video tape recorder (hereinunder referred to as "VTR").
2. Description of the Related Art
In a magnetic recording and reproducing apparatus such as a VTR, a movable head is used. The movable head is a head which is displaced in accordance with a deflection signal which is supplied from a driving control device. In a VTR, the movable head is provided in such a manner that the end of the movable head projects from the peripheral surface of a rotary drum. By driving the movable head in the state in which a magnetic tape passes over the peripheral surface of the rotary drum, the magnetic head is enabled, for example, to follow a track which is formed on the magnetic tape.
FIG. 46 shows an example of the structure of a movable head. The movable head is composed of a tongue-like piezoelectric bimorph 101 which is bent in accordance with the voltage applied thereto and a magnetic head 103 which is disposed at the free end of the bimorph 101. The magnetic head 103 is fixed to the bimorph 101 and the other end of the bimorph 103 is fixed to the inside of a rotary head (not shown). FIG. 46, and later-described FIGS. 49 and 50 show the positions of a bimorph and a magnetic head on a rotary drum. In these drawings, the circle at the back of the bimorph and the magnetic head represents the outline of the rotary drum.
In FIG. 46, a sensor 102 is a piezoelectric generator formed by cutting a part of the bimorph 101.
When a voltage is applied to the bimorph 101 and the bimorph 101 is warped in accordance with the voltage, the bimorph 101 assumes the state shown in FIG. 47. The angle .theta. (deg) shown in FIG. 47 is called the amount of inclination of the magnetic head 103 and .xi. is called the displacement of the magnetic head 103.
FIG. 48 shows the relationship between the amount of inclination .theta. (deg) of the magnetic head 103 and the effective length of the bimorph 101. As shown in FIG. 48, as the effective length of the bimorph 101 becomes longer, the amount of inclination of the magnetic head 103 reduces. As the amount of inclination of the magnetic head 103 becomes smaller, the end surface of the magnetic head comes into better contact with the magnetic tape which passes over the peripheral surface of the rotary drum, so that it is possible to record or reproduce a high-frequency signal with higher accuracy. Especially, in the case in which it is necessary to greatly displace the magnetic head 103, that is, it is necessary to increase .zeta. as at the time of superior reproduction of a VTR, it is preferable that the amount .theta. (deg) of inclination of the magnetic head 112 is small, that is, the bimorph 101 has a long effective length.
The structure of a bimorph having a longer effective length is shown in FIG. 49. In FIG. 49, a bimorph 201 is comparatively long and it is disposed obliquely relative to the radius of the rotary head. The effective length (the projected length in the right and left direction in the drawing) of the bimorph 201 is therefore longer than that of the bimorph 101 shown in FIG. 46.
The structure of a bimorph having an even longer effective length is shown in FIG. 50. In FIG. 50, an annular bimorph 301 is adopted. The effective length (the projected length in the right and left direction in the drawing) of the bimorph 301 is therefore longer than that of the bimorph 201 shown in FIG. 49.
On the other hand, if the effective length of a bimorph is long, the serial resonance frequency and the parallel resonance frequency become low. FIG. 51 shows an example of the frequency characteristic of a bimorph. In a general frequency characteristic of a bimorph, the phase reverses by 180.degree. when the frequency reaches the primary serial resonance frequency (the frequency at the lower serial resonance point of the two shown in FIG. 51). For this reason, in a VTR system in which tracking is carried out by a movable head, the control frequency band is set at a frequency band lower than the primary serial resonance frequency. If the interval between the primary serial resonance frequency and the secondary serial resonance frequency or the interval between the primary serial resonance frequency and the parallel resonance frequency is long, the control frequency band is set at a frequency band between the primary serial resonance frequency and the secondary serial resonance frequency or between the primary serial resonance frequency and the parallel resonance frequency by phase advancing control. Accordingly, when the effective length of a bimorph is long, it is difficult to secure a sufficiently wide control frequency band, thereby making sufficient tracking difficult. For example, if a track formed on the magnetic tape is rolled, it is difficult for the magnetic head to follow the rolling.
Such a defect can be ameliorated by reducing the peak gain at the serial resonance frequency to a certain extent. By differentiating the output of the sensor, it is possible to reduce the peak of the gain at the serial resonance frequency which is contained in the output.
It is also possible to use an actuator having the structure shown in FIG. 52 in order to drive the magnetic head. The structure of the actuator shown in FIG. 52 will now be explained and the structures of conventional control devices will next be explained.
The actuator shown in FIG. 52 displaces a magnetic head at a large amplitude without inclining the magnetic head. Two magnets 401 and 402 are vertically provided in the actuator, and a yoke 403 is disposed between the magnets 401, 402. An annular yoke 404 is further provided with a gap between the annular yoke 404 and the yoke 403. On the upper side and the lower side of the yoke 404 are provided yokes 405 and 406, respectively. These yokes form a magnetic path for the magnetic fluxes produced by the magnet 401 or 402.
In this actuator, the magnet 401, the yoke 403 and the magnet 402 are disposed within a bobbin 408 which has an actuator coil 407 wound therearound. The actuator coil 407 is situated between the yokes 403 and 404, namely, within the gap. Therefore, when a current flows on the coil 407, a force is applied to the coil 407 in the vertical direction in the drawing. The direction in which the bobbin 408 is moved by this force is regulated by gimbal springs 409 and 410, and the bobbin 408 vertically moves.
Each of the gimbal springs 409, 410 is composed of a discal thin metal sheet provided with a plurality of arcuate slits (not shown). At the centers of the gimbal springs 409, 410, holes are formed, and the bobbin 408 is fitted in the holes so as to be fixed on the inner peripheral edges of the gimbal springs 409, 410. The outer peripheral edges of the gimbal springs 409, 410 are fixed to the yokes. The gimbal springs 409, 410 are parallel to each other.
One 410 of the gimbal springs is integrally formed with a leaf spring 411. The leaf spring 411 is fixed to the bobbin 408 and the free end thereof is fixed to a magnetic head 412. Therefore, the actuator shown in FIG. 52 can displace the magnetic head 412 at a large amplitude without inclining the magnetic head 412.
(1) First example of a conventional device PA0 (2) Second example of a conventional device PA0 (3) Third example of a conventional device PA0 (4) Fourth example a conventional device PA0 (5) Control of the height of a magnetic head by an actuator PA0 (6) Fifth example of a conventional device PA0 (7) Sixth example of a conventional device PA0 (8) Seventh example of a conventional device PA0 (9) Eight example of a conventional device PA0 (10) AC magnetic field generating means
FIG. 53 shows the structure of a conventional movable head position controlling device for magnetic recording and reproducing apparatuses which is described in Japanese Patent Laid-Open No. Sho. 52-117107. The structure of the device will now be explained on the assumption that the actuator shown in FIG. 52 is used and wobbling servo control is executed.
In this device, the output of a sensor 502 which is formed by cutting a part of a bimorph 501 is a signal which indicates the instantaneous value of the position of a magnetic head 512. The phase of this signal is 90.degree. delayed with respect to the signal for driving the bimorph 501 (the output signal of a driving amplifier 510). A high-impedance amplifier 503 is an amplifier having a high input impedance, and the output of the sensor 502 is amplified by the high-impedance amplifier 503. The sensor 502 is equivalent to a series circuit of a capacitor and a voltage source as viewed from the input terminal of the high-impedance amplifier 503. Since the input impedance is high, the high-impedance amplifier 503 does not constitute a load to the sensor 502.
A differentiator 505 is provided via an adder 504 at a subsequent stage to the high-impedance amplifier 503, and the output of the adder 504 is differentiated by the differentiator 505. Since the output of the high-impedance amplifier 503 indicates the instantaneous value of the position of the magnetic head 512, the result of the differentiation is a signal which indicates the instantaneous value of the speed of the magnetic head 512. The frequency characteristic of the differentiator 505 is a phase advancing characteristic such as that of a high-pass filter. The differentiator 505 can be utilized for reducing the peak gain at a serial resonance frequency to a certain extent. That is, it is possible to reduce the peak of the output of the adder 504 by differentiation.
To the differentiator 505, a low-pass filter 506, a phase advancing circuit 507, a variable gain amplifier 508 are connected in series. The cutoff frequency of the low-pass filter 506 is set so as to attenuate a signal produced by the secondary and higher-order resonance frequencies of the bimorph 501 from among the outputs of the differentiator 505. When a signal passes through the low-pass filter 506, the phase of the signal delays. The phase advancing circuit 507 has a function of advancing the phase of the output of the low-pass filter. The phase advancing circuit 507 compensates for the phase delay in the vicinity of the serial resonance point of the bimorph 551. As a result, of the signals output from the phase advancing circuit 507, the frequency component in the vicinity of the serial resonance point of the bimorph 551 has a phase of clear 0.degree.. The variable gain amplifier 508 is adjustable in order to correspond to the nonuniformity in the characteristic of the bimorph 501.
The output of the variable gain amplifier 508 is input to the driving amplifier 510 via an adder 509. The driving amplifier 510 amplifies the output of the adder 509 and supplies the amplified output to the bimorph 501 as a deflection signal. In this way, the bimorph 501 is driven in accordance with the output of the driving amplifier 510. The above structure can be said to constitute a feedback loop for controlling the driving of the bimorph 501. In other words, by supplying a deflection signal to the bimorph 501, the resonance vibration of the bimorph 501 is suppressed.
The adder 504 between the high-impedance amplifier 503 and the differentiator 503 adds the output of the high-impedance amplifier 503 and the output of a potentiometer 511 and supplies the sum to the differentiator 505. The differentiator 505 fetches the output of the adder 509. It is in order to suppress the frequency component in the vicinity of the parallel resonance point of the bimorph 501 (zero adjustment) that the output of the high-impedance amplifier 503 and the output of a potentiometer 511 are added. Such suppression is possible because the deflection signal is supplied to the bimorph 501, so that the phase of the signal output from the sensor 502 is shifted by 180.degree. at the parallel resonance point. By utilizing the potentiometer 511 in this way, the servo system is stable in the vicinity of the parallel resonance point.
In this structure, however, that is, in the structure in which the peak gain at the serial resonance point is suppressed by the differentiator 505, since the noise contained in the deflection signal, which is the output of the sensor 502, is also amplified, it is impossible to secure a large loop gain of the loop for controlling the bimorph 501.
As a result, there is a trade-off relationship between the good contact between the magnetic head 512 and the magnetic tape, and the sufficient frequency band and loop gain for control. In this way, there is a certain limitation to damping control.
FIG. 53 also shows the structure of a wobbling servo system. The output of the magnetic head 512 is supplied not only to a video signal processor 513 but also a wobbling servo system which is composed of a head position controller 514, a frequency compensator 515 and a converter reset signal generator 516. The output of the wobbling servo system is supplied to the adder 509.
Wobbling servo control is a control for constantly maximizing the amplitude of a reproduction signal from the magnetic head 512. As a tracking control system, a pilot system and a wobbling system are conventionally adopted. In this conventional device, a wobbling system is adopted.
In a pilot system, a multiplicity of pilot signals for tracking servo control are recorded on the same recording track or a multiplicity of pilot signals which belong to the gap between the recording signal frequency allocation are recorded on different tracks so that signals recorded thereon are different from each other. In this system, the levels of the signals obtained as a crosstalk from the adjacent tracks are compared at the time of reproduction and the direction and the amount of off-track are detected. The results of detection are used for the control of the position of the magnetic head.
In a wobbling system, the magnetic head is forcibly vibrated minutely at the time of recording, and the frequency component of the minute vibration which is contained in the reproduction envelope signal is synchronously detected in accordance with magnetic head vibration commanding information. As a result of the synchronous detection, the direction and the amount of off-track are detected.
To state this more explicitly, in a wobbling system, the object of control is an electromagnetically driven actuator 518 having a structure in which the magnetic head 512 is driven when a current is supplied to the actuator 517, as shown in FIG. 54. The wobbling servo system includes a head amplifier 519 for amplifying the output of the magnetic head 512, a wobbling servo circuit 520 for executing wobbling servo control in accordance with the signal which is amplified by the head amplifier 519, and a driver 521 for supplying a current to an actuator coil 517 in accordance with the output of the wobbling servo circuit 520. In the circuit shown in FIG. 53, the function of the wobbling servo circuit 520 is chiefly assigned to the frequency compensator 515.
In the circuit shown in FIG. 53, the output of the magnetic head 512 is first processed by the head position controller 514. The head position controller 514 generates a tracking correction signal from the output of the magnetic head 512. The frequency compensator 515 compensates the frequency in accordance with the output of the head position controller 514 and the output of the converter reset signal generator 516. The converter reset signal generator 516 outputs a signal for moving (resetting) the magnetic head 512 to the tracking starting position. The adder 509 adds the output of the variable gain amplifier 508 and the output of the frequency compensator 515, and supplies the sum to the driving amplifier 510 and the potentiometer 511.
In this way, in this conventional device, good tracking is possible while maintaining a good contact between the magnetic head and the magnetic tape. This is because the wobbling servo system is adopted.
The conventional device having the above-described structure, however, suffers from problems such as the limitation in the control frequency band and the oscillation of the tracking servo system as follows.
When the magnetic head is driven by the actuator having the structure shown in FIG. 52, the phase reversal is caused due to the mechanical resonance of the gimbal springs and the leaf spring. The frequency characteristic of this actuator is such as that shown in FIG. 55, and the phase is reversed by 180.degree. at a frequency above the primary serial resonance frequency. Therefore, the control frequency band must be set at a frequency band sufficiently lower than the primary serial resonance frequency. If the control frequency band is raised beyond this limitation, the phase margin of the servo system is deteriorated due to the phase reversal caused by the mechanical resonance and the gain margin is also deteriorated due to the peak gain produced by the mechanical resonance. As a result, the tracking system is oscillated.
Such problems may be solved by raising the mechanical resonance frequency. In this case, however, the thickness of the gimbal is increased, so that it is necessary to increase the force constant of the actuator, which inconveniently leads to the increase in the dimension of the actuator.
Head structure
FIG. 56 is a sectional view of the main part of a conventional magnetic recording and reproducing apparatus and FIG. 57 shows the same apparatus with a seat removed therefrom, as viewed in the direction indicated by the arrows A--A in FIG. 56.
In these drawings, the reference numeral 601 represents a fixed drum, 602 a bearing attached to the fixed drum 601, 603 a rotary shaft which is rotatably supported by the bearing 602, 604 a seat fitted over one end of the rotary shaft 603 and 605 a rotary drum attached to the seat 604 by a screw 606. The reference numeral 607 represents an actuator attached to the rotary drum 605 by a screw 608, 609 a lower transformer attached to the fixed drum 601, 610 an upper transformer attached to the seat 604, 611 a distribution board, 612 a contact which does not rotate and which supplies a control current to the actuator 607, 613 rotary electrode which is provided at a part of the seat 604 in such a manner as to come into sliding contact with the contact 612, and 614 and 615 connecting portions. The rotary electrode 613 is electrically connected to the actuator 607 through the connecting portions 614, 615 and the distribution board 611. The reference numeral 616 represents a magnetic head (hereinunder referred to as "movable head") attached to the actuator 607. The movable head 616 is electrically connected to an actuator controller through a connecting portion 617, the distribution board 612 and the connecting portion 615. The reference numeral 618 denotes a recessed portion provided at a part of the rotary drum 605 for receiving the actuator 607. The recessed portion 618 is made larger than the actuator 607 so as to allow the position control of the movable head 616. A plurality of holes 619 used for the position control of the movable head 616 are provided in the recessed portion 618. The reference numeral 620 represents a magnetic tape which passes over the outer peripheral surface of the rotary drum 605 and comes into sliding contact with the movable head 606 during travelling.
FIG. 58 is a plan view of the actuator 607, FIG. 59 shows the same actuator 607, as viewed in the direction indicated by the arrows B--B in FIG. 58, and FIG. 60 is a sectional side elevational view thereof, as viewed in the direction indicated by the arrows C--C in FIG. 58. The reference numeral 621 represents a first yoke composed of a magnetic material and 622 a first permanent magnet which is columnar and fixed to the first yoke 621. A second yoke 623 composed of a magnetic material and provided with a convex portion 623b at a part of the inner periphery thereof is attached to the first yoke 621. A third yoke 624 composed of a magnetic material is attached to the second yoke 623. The reference numeral 625 represents a second permanent magnet which is columnar and fixed to the third yoke 624 with the same magnetic pole opposed to each other, and 626 a pole piece composed of a magnetic material and fixed to either the second permanent magnet 625 or the first permanent magnet 622 at an intermediate position therebetween. A leaf spring 627 is composed of a thin nonmagnetic material. The peripheral edge of the leaf spring 627 is clamped between the first yoke 621 and the second yoke 623, and the extending portion 627a thereof projects outward through windows 621a, 623a provided on the first yoke 621 and the second yoke 623, respectively. The movable head 616 is attached to the end of the extending portion 627a. A leaf spring 628 composed of a thin nonmagnetic material is clamped between the second yoke 623 and the third yoke 624. Fixing members 629 are held by the leaf springs 627 and 628. A bobbin 630 is fixed to the fixing members 629 by an adhesive 632 with a gap between the inner periphery of the bobbin 630 and the outer peripheries of the first permanent magnet 622, the second permanent magnet 625 and the pole piece 626. The reference numeral 631 denotes a coil composed of an electric wire coated with a coating material and wound around the bobbin 630. The bobbin 631 is accommodated in the annular gap G formed by the convex portion of the second yoke 623.
FIG. 61 shows the magnetic head mounted on the rotary drum 605 in the case of being used for a magnetic tape unit in accordance with the present VHS format. The movable head 616 is used as a pair of magnetic heads exclusively for superior reproduction mode (mode for fast forwarding or slowly reproducing the recorded video information). The reference numeral 635 represents a pair of EP heads for a long-time mode for recording video information on a narrow track on the video tape for a long time, 636 a pair of SP head for recording and reproducing normal video information on and from a wide track, 637 a pair of audio heads for recording and reproducing audio information, and 638 a pair of FE heads for erasing the recorded information for each track at the time of recording new information thereon.
Control system
FIG. 62 is a block diagram of a conventional control system and FIG. 63 is a perspective view of the magnetic field generator of the conventional device. In these drawings, the reference numeral 640 represents an AC magnetic field generator for supplying two magnetic fields B.sub.f1, B.sub.f2 having different frequencies from each other to the movable head 616. The AC magnetic field generator 640 is disposed at a position along the peripheral surfaces of the rotary drum 605 and the fixed drum 601 on the opposite side to the magnetic tape 620, and the position is controllable. The AC magnetic field generator 640 is provided with AC magnetic field generating coils 645, 645a arranged in the axial direction of the rotary drum 605 so as to generate the magnetic fields B.sub.f1, B.sub.f2 having different frequencies f.sub.1, f.sub.2 from each other. The reference numeral 642 denotes a band-pass filter for passing the frequency component f.sub.1 therethrough and 643 a band-pass filter for passing the frequency component f.sub.2 therethrough.
FIG. 64 is a circuit diagram of a third example of a conventional device. The reference numeral 746 represents a driver for supplying a current to a coil 745, 747 an oscillator for generating an AC voltage, 748 and 749 rotary transformers for transmitting and receiving a signal to and from a magnetic head in a rotary drum 705, and 750 and 751 recording/reproduction signal amplifiers for amplifying signals from an audio head and a video head and supplying a recording current. The reference numeral 752 denotes a band-pass filter for only passing therethrough a signal which is electromagnetically induced by the oscillating coil 745 and reproduced by an audio head 737 fixed in the rotary drum 705, and 753 a band-pass filter for only passing therethrough a signal which is electromagnetically induced by the oscillating coil 745 and reproduced by a movable head 716. A sample hold circuit 755 holds the output which is reproduced by the movable head 715 at every other rotation of the rotary drum 705, electromagnetically induced by the oscillating coil 745 and amplified. A differential amplifier obtains the difference between the signals of the sample hold circuits 754 and 755 and has the characteristic shown in FIG. 756. The reference numeral 757 denotes a servo compensator composed of a low-pass filter or the like so as to secure the stability in a position fixing control loop, and 758 a driver for supplying a driving current to an actuator 707.
FIG. 66 is a sectional view of the AC magnetic field generating coil 745. The reference numeral 745c represents a magnetic core for concentrating magnetic fluxes generated by the coil 745, 745U a coil for generating AC magnetic fluxes from the AC current flowing thereon, 745L a coil for generating AC magnetic fluxes in the reverse direction to that generated by the coil 745U, 745b a coil holder for accommodating the coils 745U, 745L, and 759 a fixing member for fixing the AC magnetic field generating coil 745. FIG. 67 shows the directions of the magnetic fluxes generated by the AC magnetic field generating coil 745.
FIG. 68 is a circuit diagram of a fourth example of a conventional device and FIG. 69 is an enlarged view of the part A in FIG. 68. Two AC magnetic field generating coils 845, 845a, which are so designed as to be insusceptible to the influence of each head such as nonuniformity in the sensitivity, are disposed in the peripheral direction of a rotary drum 805. The reference numeral 850 denotes a first divider for obtaining the ratio of the amplitudes of the outputs of the two AC magnetic field generating coils 845, 845a which are reproduced by a fixed head, and 860 a second divider for obtaining the ratio of the amplitudes of the outputs of the two AC magnetic field generating coils 845, 845a which are reproduced by a movable head 816.
The operation of the actuator 607 in the second example of a conventional device will be explained with reference to FIGS. 56 to 63.
The first permanent magnet 622 generates magnetic fluxes D by the closed magnetic path formed by the pole piece 626, the second yoke 623 and the first yoke 621.
Similarly, the second permanent magnet 625 generates magnetic fluxes E in the direction opposite to that of the magnetic fluxes D by the closed magnetic path formed by the pole piece 626, the second yoke 623 and the third yoke 624.
Both the magnetic fluxes D and the magnetic fluxes E generated in this way intersect the annular gap G in the same direction, and the total magnetic fluxes generated by the first permanent magnet 622 and the second permanent magnet 625 cross the coil 631.
In this state, when a current is caused to flow on the coil 631 from the contact 612 through the electrode 613 and the connecting portions 615, 614, the bobbin 630 and the movable head 616 are integrally moved in the vertical and axial direction.
Consequently, the movable head 616 is displaced in the direction of the width of the magnetic tape 620 and traces the recorded track with high accuracy.
FIG. 70 shows the hysteresis characteristic between the driving current for the magnetic head actuator 607 and the amount of displacement of the movable head 616, and FIG. 71 shows the recording tracking pattern on the magnetic tape 20 at the time of recording signals by using the magnetic head actuator 607 having the hysteresis characteristic shown in FIG. 70.
As is clear from FIGS. 70 and 71, in the case in which the magnetic head actuator 607 is controlled only at the initial stage, the reference position of the magnetic head 616 varies due to the hysteresis characteristic shown in FIG. 70, so that the recorded tracks overlaps by .alpha..
The movable head 616 detects the magnetic fields B.sub.f1, B.sub.f2 generated by the AC magnetic field generating coils 645, 645a, respectively, every time the movable head 616 passes the vicinity of the AC magnetic field generator 640, and outputs the detection signal which is proportional to the intensity of the magnetic field. The band-pass filter 642 transmits the signal component S having a frequency of f.sub.1 and the band-pass filter 643 transmits the signal component T having a frequency of f.sub.2
The levels of these two signal components S, T vary when the movable head 616 is moved in the axial direction of the rotary drum 695, in other words, with a change in the height of the movable head 616, as shown in FIG. 72. If it is assumed that the height of the movable head 616 at which the two signal components S, T are at the same level is magnetic and the level of the signal components at that time is l, a subtracter 644 obtains the difference between the signal components S and T, feeds back the difference signal to the actuator 607 and moves the movable head 616 in the direction in which the difference becomes zero. In other words, the movable head 616 is moved so that the levels of the signal components S and T are the same, that is, the height of the movable head 616 is magnetic in FIG. 72. Since it is possible to vary the intersection l of the signal components S, T and vary the height magnetic of the movable head 616 by varying the positions or the like of the AC magnetic field generating coils 645, 645a, it is possible to freely determine the reference position of the movable head 616.
Although the control of one movable head is explained in the second example, it is possible to cancel the difference in head level between channels at the time of recording by an apparatus provided with a plurality of movable heads by controlling each movable head in the same way.
As shown in FIG. 73, the AC magnetic fluxes having a frequency of f.sub.1 generated by the two coils 645U, 645L of the AC magnetic field generating coil 645 repel each other at the position at which the coils 645U and 645L are opposed to each other, and a region in which the flux density is high and a region in which the flux density is low are formed in the vertical direction.
The AC magnetic flux is reproduced by a reproduction signal amplifier 650 or 651 through a rotary transformer 648 or 649 when the movable head 616 or the audio head 637 passes the AC magnetic field. At this time, the oscillation frequency f.sub.1 of the oscillator 647 is set at a frequency higher than the attenuation-frequency limit of the rotary transformers 648, 649 defined by the frequency characteristic on the low frequency side and lower than the frequency at which the driving current becomes difficult to supply due to the inductance of the AC magnetic field generating coil 645. The attenuation-frequency limit of the rotary transformers 648, 649 is generally several 10 KHz to 100 KHz. For example, if the number of turns of the coils 645U, 645L is several hundred and the frequency at which attenuation start is 1 MHz, the oscillation frequency f.sub.1 is determined set as, for example, 100 KHz&lt;f.sub.1 &lt;1 MHz.
The operation of the second example of a conventional device will be explained with reference to FIGS. 64 to 67.
In FIG. 64 and 66, when the magnetic head 716 or the audio head 737 passes the vicinity of the AC magnetic field generating coil 745, the amplitude of the reproduction signal having a frequency of f.sub.1 and output from the reproduction signal amplifier 750 or 751 is increased by moving the movable head 716 upward (away from the deck base) and reduced by moving the movable head 716 downward in the case in which the intermediate position between the two coils 745U and 745L is higher than height of the audio head 737 or the fixed height of the movable head 716 at the neutral position. If the intermediate position between the two coils 745U and 745L are lower than the height of the audio head 737 or the fixed height of the movable head 716 at the neutral position, the direction of attenuation of the reproduction signal is reversed. It is now assumed that the detection sensitivity for the signal output from the reproduction signal amplifier 750 as the reproduction signal from the fixed head (audio head) 737 is equal to the detection sensitivity for the signal output from the reproduction signal amplifier 751 as the reproduction signal from the movable head 716 or they are adjusted to be equal by the gain adjustment of the reproduction signal amplifier 750 or 751. The outputs of the reproduction signal amplifiers 750, 751 are passed through the band-pass filters 752, 753, respectively, which transmit only the frequency f.sub.1 and unnecessary noise is removed therefrom. These two outputs are supplied to the sample hold circuits 754 and 755 for sample holding or peak holding so as to obtain the maximum levels thereof and thereafter the difference in level is taken out by the differential amplifier 756 so as to obtain the difference in height between the movable head 716 and the fixed head 737 as a function of a voltage. The difference obtained is passed through the phase compensator 757 such as a low-pass filter in the control system, and the control loop is closed in the direction in which the difference in head level is cancelled. Thus, the movable head 716 and the fixed head 737 are held such that there is no difference in head level at the time of recording, either.
Similarly, in the case in which two movable heads 716 are mounted on the rotary drum 705 at the diametrically opposite positions, it is possible to cancel the difference in the head level between channels by providing the above-described head height fixing control system in each actuator.
In this case, the servo band of the position fixing control loop need not be very wide because the position fixing control system only corrects the difference in height between the movable head 716 and the fixed head 737 or difference in height between two movable heads 716. Since the difference in height or level of head is detected at every other rotation of the rotary drum 705, if the rotary drum 750 rotates at a rate of 1,800 rpm, the time corresponding to 30 Hz is wasted by sampling. Therefore, unless the control frequency band is set at not more than several Hz, the control system oscillates. For this reason, the compensator 757 determines the time constant and the gain so that the control frequency band becomes several Hz and the phase gain is not less than 60 deg.
Returning to the second example, it goes without saying that in the head level control system, while the movable head 616 is passing over the magnetic tape along the peripheral surfaces of the drums 601, 605, the recording/reproduction signal amplifier functions as a recording signal amplifier, and while the movable head 610 moved in the vicinity of the AC magnetic field generating coil 645 on the opposite side of the drums 601, 605 with respect to the magnetic tape 620, the recording/reproduction signal amplifier functions as a reproduction signal amplifier.
The head level control system has the above-described structure. In the conventional device shown in FIG. 64, the detection sensitivities of the heads 716, 737 and the reproduction signal amplifiers 750, 751 must be equal to each other or adjusted to be equal. Actually, however, it is often difficult to equally adjust the sensitivities due to the difference in the number of turns between the fixed head 737 and the movable head 716, the difference in magnetic permeability between the head cores, the nonuniformity in the gains of amplifiers 750, 751 and the difference in the temperature characteristics.
As a countermeasure, in an conventional device shown in FIG. 68, two AC magnetic field generating coils 845 and 845a having different oscillation frequencies f.sub.1 and f.sub.2 are provided and one AC magnetic field generating coil 845 is fixed such that the intermediate height between the two coils is higher than the height of a fixed head 837 and the intermediate height between the two coils of the other AC magnetic field generating coil is lower than the height of the fixed head 837. At this time, if the height of a movable head 816 is so controlled that the amplitude ratio of the output having a frequency of f.sub.1 which is electromagnetically induced by the oscillating coil 845 and reproduced by the fixed head 837 and the output having a frequency of f.sub.2 which is electromagnetically induced by the oscillating coil 845a and reproduced by the fixed head 837 is equal to the amplitude ratio of the outputs of the movable heads 816, it is possible to cancel the difference in height between the movable head 816 and the fixed head 837 irrespective of the difference in the number of turns between heads, the difference in magnetic permeability between the head cores, the nonuniformity in the gains of amplifiers and the difference in the temperature characteristics so long as the frequency characteristics of the fixed head system and the movable head system are not greatly deviated from the frequency characteristic of each of the heads and reproduction signal amplifiers at the frequencies f.sub.1 and f.sub.2. The output reproduced by the movable head 816 is input to band-pass filters 853, 853a for transmitting only the frequencies f.sub.1 and f.sub.2, respectively, and the amplitudes of the reproduced signals are input to a divider 860 through sample hold circuits (or peak hold circuits) 855, 855a and taken out as a division signal. Similarly, the output reproduced by the fixed head 837 is input to band-pass filters 852, 852a for transmitting only the frequencies f.sub.1 and f.sub.2, respectively, and the amplitudes of the reproduced signals are input to a divider 859 through sample hold circuits 854, 854a and taken out as a division signal. The difference between these division signals is obtained by a differential amplifier 856, whereby it is possible to detect the direction and the amount of deviation of the height of the movable head 816 from the height of the fixed head 837. For example, if the top of the movable head 816 is situated at a position higher than the top of the fixed head 837 (the movable head 816 deviates from the fixed head 837 in the direction which is away from the deck base), in the reproduction signal of the movable head 816, the amplitude of the frequency component f.sub.1 is larger than that of the frequency component f.sub.2 as compared with the reproduction signal of the fixed head 837. Consequently, the output signal of the differential amplifier 856 is negative, so that the movable head 816 is moved down to the position at which there is no difference in head level.
In the above-described manner, accurate head level control is executed even if there is nonuniformity in sensitivity between the heads 816, 837 or the head amplifiers 850, 851. The conventional device shown in FIG. 68, however, requires the highly accurate dividers 859, 860, which may lead to rise in cost.
FIG. 74 is a circuit diagram of a fifth example of a conventional device, which does not use a divider. The reference numeral 961 represents a switching circuit, 962 a a timing controller for controlling the holding timings of sample hold circuit 955, 955a.
In the fifth example, the output of a fixed head 937 which is amplified by a reproduction signal amplifier 950 and further the outputs of band-pass filters 952, 952a for transmitting only the frequencies f.sub.1, f.sub.2 are so controlled that the amplitudes of the output signals having frequencies of f.sub.1 (=150 KHz) and f.sub.2 (=200 KHz), respectively, are equal by adjusting the positions at which AC magnetic field generating coils 945, 945a are provided and the driving voltages of the drivers 946, 946a by adjusting terminals while observing the output level of the reproduction signal. By controlling the height of a movable head 916 so that the amplitudes of the reproduction signal components having frequencies of f.sub.1, f.sub.2 are equal in this way, it is possible to eliminate the difference in level between the movable head 917 and the fixed head 937 without using a divider.
In this example in which the two movable heads 916 are mounted on a rotary drum at diametrically opposite positions, the control of these heads is executed by distributing a reproduction signal of each channels to four sample hold circuits 955, 955a by the analog switching circuit 961 which is provided at the subsequent stage to band-pass filters 953, 953a. In this case, two differential amplifiers 956, 956a, two compensators 957, 957a and two drivers 958, 958a are necessary at the subsequent stages. Such correspondence to multi-channels are also applicable to the conventional devices shown in FIGS. 64 and 68. The control frequency band is set in the conventional device shown in FIG. 74 in quite the same way as in the conventional devices shown in FIGS. 64 and 68, and the gain and the phase is compensated for by a compensator. Since a magnetic head generally picks up a magnetic flux in the direction to which the circumference of the rotary drum is connected, if the AC magnetic field generating coils have the configurations shown in FIG. 66, the outputs are taken out in reproduction envelopes such as those shown in FIGS. 75 to 77. In the case of the structure shown in FIG. 74, since the reproduction signal of the fixed head 937 is adjusted so that f.sub.1 and f.sub.2 are equal, the output signal takes the form shown in FIG. 75. Even if the sensitivity between the head and the head amplifier is deviated from that of the movable head system, when the levels of the frequency components f.sub.1 and f.sub.2 becomes equal after the control of the movable head 916, as shown in FIG. 77, the difference in head level is cancelled.
FIG. 78 is a circuit diagram of a sixth example of a conventional device. This example has a structure similar to that which is generally used by a minute displacement gauge. An AC magnetic field generating coil 1045 is disposed such that the intermediate height of the two coils thereof is the same as the height of a movable head 1016. When the movable head 1016 is deviated in the vertical direction, the direction and the amount of deviation of head level is detected by detecting the deviation of the amplitude and the phase shift by a synchronous detector 1063, as shown in FIG. 79. In this case, the processing after sample holding the synchronous detection signals is the same as in the conventional devices shown in FIGS. 64, 68 and 74. In this way, if it is possible to control the difference in level between the movable head 1016 and the fixed head 1037 so as to be constantly cancelled at the time of recording, it is unnecessary to attach a fixed head exclusively for recording to a rotary drum 1005, and recording, reproduction and superior reproduction of a video signal, for example, can be assigned to the movable head 1016 mounted on an actuator 1007. In addition, since it is possible to adjust the level of the movable head 1016 to the level of the fixed head 1037, it is possible to mount a Hi-Fi audio head 1037 for a VHS format and an erase head 1038 for recording new information on the recorded track on the rotary drum 1005 and to mount an EP head 1035 and an SP head 1036 on the actuator, as shown in FIG. 80. It is thus possible to greatly simplify the arrangement of heads in comparison with the arrangement shown in FIG. 61.
In the example shown in FIG. 81, the amplitudes of the reproduction signals having frequencies of f.sub.1 and f.sub.2, respectively, which are output from the fixed head 1037, are adjusted to be equal by adjusting the mounting position of the AC magnetic field generating coils 1045 and the driving voltage levels. However, it is sometimes impossible to make those amplitude equal merely by adjusting the mounting position of the AC magnetic field generating coils 1045 or the driving voltage levels, or the initial adjustment solely is sometimes insufficient for practical use due to temperature characteristics, change with time, etc.
FIG. 81 is a circuit diagram of a seventh example of a conventional device. In this example, an AC magnetic field control system is provided so as to electrically adjust the amplitudes of the reproduction signals of a fixed head 1137 to be equal when they cannot be made equal by the adjustment of the mounting position of an AC magnetic field generating coil 1145. The reference numeral 1165, 1165a represent variable gain control amplifiers for controlling the level of the AC magnetic fields generated by coils 1145 and 1145a.
In this example, the amplitudes of the output signals having frequencies of f.sub.1 and f.sub.2, respectively, which are output from a fixed head 1137 and passed through band-pass filters 1152, 1152a, are adjusted to be constant by inputting the outputs of sample hold circuits 1154, 1153a to the gain control input terminals of the variable gain control amplifiers 1165, 1165a, respectively. In this way, the amplitude is so controlled as to be constant (in this case, the reproduction outputs of the fixed head 1137 having frequencies f.sub.1, f.sub.2 are so controlled as to have equal amplitudes) irrespective of variation of the mechanical position control of the AC magnetic field generating coils 1145, 1145a, temperature characteristics, change with time, etc.
FIG. 82 is a circuit diagram of an eighth example of a conventional device. This device has the same structure as the device shown in FIG. 81 except that the control of the magnetic field level is executed by adjusting only one AC magnetic field generating coil 1245a. The reference numeral 1266 represents a differential amplifier.
In this example, the signal components having frequencies of f.sub.1 and f.sub.2 are extracted from the reproduction output of a fixed head 1237 by band-pass filters 1251, 1252a, respectively, and supplied to sample hold circuits 1254, 1254a, respectively. The differential amplifier 1266 obtains the difference between the values output from the sample hold circuits 1254, 1254a and supplies the driving voltage level of the AC magnetic field generating coil 1245a to the variable gain control amplifier 1265. In this way, the reproduction output level from the other AC magnetic field generating coil 1245 and the reproduction output level from the AC magnetic field generating coil 1245a are adjusted to be equal. This example produces a similar effect to that of the example shown in FIG. 81.
By adding the AC magnetic field control system for controlling the AC magnetic field generating coils 1245, 1245a, it is possible to maintain the accuracy of the movable head position control system shown in FIG. 74 even if there is a fluctuation in adjustment of the mounting position of the AC magnetic field generating coils 945, 945a, temperature characteristics, change with time or the like.
Although the devices shown in FIGS. 64 to 82 have analog circuit structures, it goes without saying that the output of a reproduction signal amplifier or a band-pass filter may be subjected to analog-digital conversion, and after processings such as subtraction, sample holding and compensation filtering by a digital circuit or a software in a microcomputer, the processed output may be subjected to digital-analog conversion so as to drive an actuator.
The structure of an AC magnetic field generating coil will now be explained in detail.
In order to rapidly change the flux density depending upon locations, it is first necessary to concentrate magnetic fluxes. For example, there is a method of concentrating magnetic fluxes by applying a current to coils which are opposed to each other, as shown in FIG. 67, so as to cause the coils to repel each other.
As shown in FIG. 73, magnetic fluxes are concentrated on the region between the coils. As magnetic fluxes are apart from the core, they rapidly diverge and the flux density becomes low. In this way, this method is favorable because flux density is different in locations. The change in flux density here does not refer to a change in number of magnetic fluxes at a certain position but a change in the flux density in the direction in which a movable head can detect magnetic fluxes with respect to the axial direction of the rotary drum, as described above. It is therefore necessary to investigate the direction of magnetic fluxed generated by an AC magnetic field generating coil. FIG. 83 schematically shows the coordinate plane for examining the magnetic field distribution of an AC magnetic field generating coil 1345. The reference numerals 1345U, 1345L denote coils, 1345c a magnetic core composed of a soft magnetic material such as soft iron, and 1346 an AC power source for applying a current to the two coils 1345U, 1345L. The plane A is a plane which has the center axis L of the magnetic core 1345c as the normal line and which intersects the center of the space between the two coils 1345U, 1345L. The plane B is a plane parallel to the plane A and distant therefrom by d, and the plane C is a plane parallel to the plane A and the plane B and distant from the plane B by d and from the plane A by 2d. The curved surface D is a part of the peripheral surface of a cylinder which has the center axis in the same direction as the center axis L of the magnetic core 1345c and which has a radius of R. It is assumed that the curved surface D represents a peripheral surface of a rotary drum and that the lines where the curved surface D intersect the planes A, B and C represent the loci of the movable head.
Although an AC current is actually applied to the coils 1345U, 1345L, it is assumed here to apply a DC current thereto for the convenience of explaining the principle. FIGS. 84 to 86 schematically show the vectors of magnetic flux on each plane when a DC current is applied th the coils 1345U, 1345L so as to cause the poles to repel each other. In each drawing, the circle represents the cross section of the magnetic core, and the curve X--X' a line where each plane intersects the curved surface D.
Referring first to the plane A, in the region close to the magnetic core 1345c, the vector of magnetic flux on the plane A is large, and as magnetic fluxes are apart from the core, they take a roundabout path, so that the vector of magnetic flux on the plane A is rapidly reduced.
On the plane B which is distant from the plane A by d, since magnetic fluxes take a roundabout path, the vector of magnetic flux on the plane B is at its maximum in a region which is apart from the core 1345c to a certain extent.
This phenomenon is the same with the plane C, but since magnetic fluxes take a roundabout path and the vector of magnetic flux gradually approaches zero, the absolute value of the vector is smaller than that on the plane B.
As described above, the curve X--X' on each plane represents the locus of the movable head, and the direction in which magnetic fluxes can be detected by the movable head is represented by the line connecting the points on the curve X--X'. In FIG. 87, the magnetic fluxes in FIGS. 84 to 86 are converted into AC magnetic fluxes and the curved surface D is developed into a flat plane. Each group of the arrows represents the vector of magnetic flux on the plane D on each line where the plane D intersects the corresponding plane. Since the magnetic fluxes are AC magnetic fluxes, the vector is represented by pairs of arrows pointing the opposite directions.
FIGS. 88 to 90 show the waveforms output by the induced electromotive force of the movable head which has passed the lines where the planes A, B and C, respectively, intersect the curved surface D on the assumption that the magnetic flux distribution is as shown in FIG. 87. As is obvious from the output waveforms, the peak level is different in planes. In this example, the peak level on the plane B is the highest. In other words, the peak level is a nonlinear function which depends upon the amount of displacement of the movable head in the axial direction of the rotary drum. It is therefore possible to know the absolute position of the movable head by detecting the peak level of the output waveform.
In order to control the position by the movable head as a position sensor, the AC magnetic field generating coil 1345 is disposed such that the movable head is fixed in a region in which the variation of the peak level of the output waveform is large, i.e, the region between the plane A and the plane B or the region between the plane B and the plane C in FIG. 87.
The above-explained magnetic field distribution is obtained in the case in which the AC magnetic field generating coil 1345 is driven by a specific AC voltage. The magnetic field distribution is a function which also depends upon a voltage amplitude. Therefore, by controlling the voltage so as to maximize the variation of the peak level of the output waveform with respect to the height of the head, the position of the movable head can be controlled.
The AC magnetic field generating coil 1345 provided in a drum deck in this way may exert a deleterious influence such as generating noise in the linear audio head and erasing information on the magnetic tape. A method of shielding magnetism by covering a part of a magnetic field generating element with a soft magnetic material 1345s, as shown in FIG. 91 is known. FIG. 92 shows the soft magnetic material, as viewed in the direction indicated by the arrows Y--Y. shown in FIG. 91. The above-described deleterious influence is prevented in this way.
The AC magnetic field generating coil 1345 in this example has the structure shown in FIG. 83 so as to concentrate magnetic fluxes. Alternatively, it may have a structure such as those shown in FIGS. 93 and 94 although the sensitivity of the sensor is deteriorated.
In the above-described conventional devices, the mechanical accuracy for mounting the magnetic field generating coil 640, for example, in FIG. 63 is sufficient even with a change with temperature and time taken into consideration. That is, when the accuracy for mounting the coil 640 is sufficiently lower than the tolerable accuracy for fixing the position of the movable head 616, it is possible to set the height of the movable head 616 at the position determined by the mounting position of the magnetic field generating coil 640 in the above-described manner.
Even in the case in which the accuracy for mounting the coil 640 is low, when a rotary drum provided with a plurality of movable heads, as shown in FIG. 63, is used, it is possible to make the relative height of each movable head equal. (In this case, it is impossible to control the height of the head on the basis of the absolute height of the movable head 616 from the deck base which supports the rotary drum 605).
It is also possible to make the height of the movable head 616 equal to the height of the fixed head on the rotary drum.
As described above, it is possible to control the absolute height of the movable head so as to be equal to the position at which the magnetic field generating coil is mounted or to the absolute position of the fixed head, or to control the relative height of each head so as to be equal. In the present system such as the VHS format and the B format, however, there is a case in which the height of the movable head must be so controlled as not to be equal to the height of the fixed head but to be a little deviated therefrom. In addition, in other systems such as an 8-mm video and D-1 or D-2 digital VTR, if it is possible to regulate the absolute height of the movable head from the deck base to a predetermined height, it is possible to form an accurate recorded track in accordance with the corresponding tape format. If the coil mounting accuracy required with due consideration of temperature characteristics and a change with time is not so strict, the working accuracy required for coil is also lessened and the adjustment of the coil becomes easy, which leads to reduction in the cost of the system. A means for detecting the absolute height of the movable head from the deck base is therefore demanded.
In the conventional devices, it is only possible to detect the height of the head at a predetermined point at every rotation of the rotary drum. Since the movable head sometimes moves within one rotation of the drum due to a change in the tension of the magnetic tape or in the sliding friction between the tape and the head, or sometimes mechanically vibrates, it is necessary to take a measure such as reduction in the mechanical resonance by increasing the rigidity of the gimbal spring of the movable head actuator (FIG. 65).